The present invention relates to an optical head (i.e., a data recording/reproducing head) for an optical disc drive for recording, reproducing and/or erasing data on an optical disc.
Conventionally, data recording/reproducing devices for recording/reproducing data on an optical disc have been known. Among such devices, a device which divides a light beam reflected by the optical disc into a pair of beams, and generates a servo signal in accordance with the signals received by a pair of light receiving elements has been known. An example of such a device is disclosed in Japanese Patent Provisional Publication No. HEI 7-326084.
In the Publication, an optical head is constructed as follows. A reflected laser beam from an optical disc is divided into three beams using a Wollaston prism. One of the beams is used as a beam for generating a servo signal, and the other two beams are used as beams for generating a data signal. Further, using a hologram plate, the beam for generating the servo signal is divided into a pair of beams in a direction perpendicular to the direction where the Wollaston prism divides the beam, and causes the divided beams to defocus, with respect to a predetermined focal plane, in positive and negative directions, respectively. The beams for generating the servo signal that emerge from the hologram plate are incident on a pair of photo sensors. In accordance with the outputs of the servo sensors, the servo signal is obtained.
In the above-descried optical head, each of the beams for generating the data signal is also divided into a pair of beams using the hologram plate in the direction where the Wollaston prism divides the beam. The divided beams are impinged onto two pairs of photo sensors, which are located next to the sensors for the servo signal. Then, based on the output of the sensors for obtaining the data, data signal (which will be referred to as an MO signal) is obtained.
FIG. 16 shows a configuration of sensors 50, 51, 50xe2x80x2 and 51xe2x80x2 and a signal processing unit 52 for a conventional optical head. FIG. 17 is an enlarged view of one of the sensors 50, 51, 50xe2x80x2 and 51xe2x80x2. The laser beam reflected by an optical disc is divided into six beams as described above in a direction corresponding to a tracking direction on the optical disc (indicated by arrow T). It should be noted that, when a beam spot moves on the optical disc in the tracking direction, the reflected beam moves in the direction T with respect to the sensors 50, 51, 50xe2x80x2 and 51xe2x80x2. In FIG. 16, the sensors for obtaining the servo signal (which will be referred to as servo sensors) are the sensors 51 and 51xe2x80x2 which are located between the sensors 50 and 50xe2x80x2 for generating the data signal (which will be referred to as data sensors). As shown in FIGS. 16 and 17, a light receiving surface of each of the servo sensors 51 and 51xe2x80x2 are divided into three sectional light receiving surfaces 51a-51c, and 51xe2x80x2a-51xe2x80x2c, respectively.
The signal processing unit 52 includes adders 53-59 and subtractors 60-62. The adders 53-59 and the sensors 50, 51, 50xe2x80x2 and 51xe2x80x2 are connected as shown in FIG. 16, and the subtractors 60-62 and the adder 59 are connected to the adders 53-58 as shown in FIG. 16. The subtractor 60 outputs a focusing error signal (FES), the subtractor 61 outputs a tracking error signal (TES), the subtractor 62 outputs the MO signal (i.e., the data signal), and the adder 59 outputs a pre-format signal (i.e., an RO signal).
According to the conventional optical head as described above, the FES and the TES may not be affected by polarized condition of the laser beam reflected by the optical disc. Further, according to the above configuration, the number of the elements and/or portions to be adjusted is relatively small, and therefore adjustment of positions thereof can be done relatively easily.
In the above-described type of the optical head, when a beam spot traverses the tracks on the optical disc, a balance of the polarization components may change. In such a case, even if an objective lens of the optical head is in an in-focus condition with respect to a data recording surface of the optical disc, the focusing error signal may be generated, i.e., the FES is generated due to a T/F (tracking-to-focusing) cross-talk.
In the conventional optical head, adjustment has been made so that the T/F cross-talk becomes the lowest when the data recording tracks of the optical disc are formed at a predetermined pitch.
Recently, because of requirements for upsizing of capacity of the optical discs, a disc which may have a smaller track pitch may be used. In general, for a certain optical head, a wavelength of the laser beam, a numerical aperture (NA) of an objective lens, and an effective diameter of the objective lens are fixed. If the shape and/or pitch of grooves of the tracks of the optical disc are different from those which the optical head is originally designed for, a diffraction angle of the diffracted beam which is reflected by the optical disc may vary. Therefore, the quantity and/or intensity distribution of the diffraction components of the beams received by the sensors may vary.
A diffraction angle xcex8 of an m-th diffraction component of a laser beam whose wavelength is xcex is expressed as follows.
sin xcex8=xc2x1mxc2x7xcex/Tp
where, Tp represents a track pitch of an optical disc.
Therefore, the amplitude and a sensitivity of the tracking error signal (TES) and the focusing error signal (FES) generated by the sensors are affected by the variation of the track pitch Tp. Further, the T/F cross-talk is also affected by the variation of the track pitch Tp.
In the conventional optical head described above, each of the light receiving surfaces of the servo sensors 51 and 51xe2x80x2 is divided into three sections by lines extending in a direction perpendicular to the T direction in FIG. 16. Therefore, when a disc is replaced with another disc having a different track pitch, the T/F cross-talk may not be cancelled sufficiently. In practice, however, the T/F cross-talk should be sufficiently suppressed regardless of the track pitch of the optical disc.
If a performance of an optical head is adjusted for an optical disc whose track pitch is relatively small, and if the optical head is used for an optical disc having a greater track pitch, the servo signal may become unstable, which prevents a quick seek operation.
It is therefore an object of the present invention to provide an improved optical head which is capable of generating a stable servo signal, and suppressing the T/F cross-talk sufficiently regardless of the track pitch of an optical disc.
For the above object, according to the present invention, there is provided an optical head which is provided with:
a beam splitting element which divides a beam reflected by a data recording surface of an optical disc into two beams with generating opposite defocus therebetween; and a pair of light receiving elements which receive the two beams divided by the beam splitting element, respectively, the pair of light receiving elements being arranged substantially on a same plane. A light receiving surface of each of the pair of light receiving elements is divided, by two lines extending in a direction corresponding to a tracking direction on the optical disc, into at least three light receiving areas. A focusing error signal is generated in accordance with outputs of the at least three light receiving areas.
Since the focusing error signal is generated based on the outputs of the light receiving areas which are arranged in a direction perpendicular to a direction corresponding to the tracking direction, variation of the track pitch of the optical disc may affect little in generating the focusing error signal.
According to another aspect of the invention, there is provided an optical head which is provided with: a beam splitting element which divides a beam reflected by a data recording surface of an optical disc into two beams with generating opposite defocus therebetween; and a pair of light receiving elements which receive the two beams divided by the beam splitting element, respectively, the pair of light receiving elements being arranged substantially on a same plane. A light receiving surface of each of the pair of light receiving elements is divided, by at least two lines extending in a direction corresponding to a tracking direction on the optical disc, into at least three light receiving areas. Further, the light receiving surface of each of the pair of light receiving elements is divided, by another line extending in a direction that is perpendicular to a direction corresponding to a tracking direction on the optical disc, into two light receiving areas. With this configuration, a focusing error signal and a tracking error signal are generated in accordance with outputs of the at least six light receiving areas, respectively.
Also with this configuration, variation of the track pitch of the optical disc may affect little in generating the focusing error signal.
Optionally, the focusing error signal is generated in accordance with a spot size method, and the tracking error signal is generated in accordance with a push-pull method.
Further optionally, the beam splitting element may include a hologram element.
In particular, the pair of beams divided by the beam splitting element are xc2x11st order diffraction beams diffracted by the hologram element, wherein a distance xcex94L satisfies condition (1):
xcex1xe2x89xa6xcex94Lxe2x89xa63xcex1xe2x80x83xe2x80x83(1)
where, xcex1 is expressed by equation (2):
xcex1=2xc2x7└4xc2x7(NA0)2xc2x7"sgr"+3xc2x7(NA1)2xc2x7xcex94Z┘+NA1xe2x80x83xe2x80x83(2)
where, NA0 is a numerical aperture of an objective lens of the optical head, NA1 is a numerical aperture of a servo lens of the optical head, "sgr" is a deviation amount of a surface of the optical disc, and xcex94Z a distance, along an optical axis of the optical head, between converging positions of the divided beams.
According to a further aspect of the invention, there is provided an optical head which is provided with: a beam splitting element which divides a beam reflected by a data recording surface of an optical disc into two beams with generating opposite defocus therebetween; a pair of light receiving elements which receive the two beams divided by the beam splitting element, respectively.
Optionally, the pair of light receiving elements may be arranged substantially on a same plane.
Further optionally, a light receiving surface of each of the pair of light receiving elements being divided, by two lines extending in a direction corresponding to a tracking direction on the optical disc, into at least three light receiving areas.
At least a focusing error signal and deviation of wavelength of the beam reflected by the data recording surface are detected based on outputs of the pair of light receiving elements.
Optionally, the light receiving surface of each of the pair of light receiving elements may be divided, by at least two lines extending in a direction corresponding to a tracking direction on the optical disc, into at least three light receiving areas.
Still optionally, the optical head may be provided with a compensation system which compensates the focusing error signal based on the deviation of the wavelength of the beam.
With this configuration, even if the wavelength changes and the distance between beam spots changes, the accurate focusing error signal can be obtained, and therefore accurate servo control can be realized.
In particular, the compensation system may include an amplifier whose gain is changeable in accordance with the deviation of the wavelength of the beam.
According to a further aspect of the invention, there is provided an optical head which is provided with: a beam splitting element which divides a beam reflected by a data recording surface of an optical disc into two beams with generating opposite defocus therebetween; and a pair of light receiving elements which receive the two beams divided by the beam splitting element, respectively. A light receiving surface of each of the pair of light receiving elements is divided, by two lines extending in a direction corresponding to a tracking direction on the optical disc and by another line extending in a direction that is perpendicular to a direction corresponding to a tracking direction on the optical disc, into six light receiving areas, and a focusing error signal and deviation of wavelength of the beam may be detected in accordance with outputs of the six light receiving areas.
Optionally, the pair of light receiving elements are arranged substantially on the same plane.